This is Part 2 of a series on interstellar comets. Read Part 1. Once you start listing the properties of 3I/ATLAS, it becomes clear pretty quickly that this thing is distinctly different from any other comet we've ever seen. Here's just a small taste. Its gas plume contains way more nickel than we've ever detected in other comets. Metals like nickel tend to sink toward the center of a solar system during formation — they're heavy, they like to be close to the action — so finding a comet drenched in the stuff is, well, odd. It also doesn't have a lot of water, which is a problem if you're a comet. Comets are supposed to be dirty snowballs. Water is kind of their whole deal. As it got closer to the Sun, it got brighter — which makes sense, because it was warming up and reflecting more sunlight. But it brightened much more quickly than other comets do. The light curve was steeper than expected, like it had a hair trigger for solar heating.…